Lets make one thing clear.

I still get a lot of questions from people on which ab exercise they need to do to get a sixpack. And when I tell them to do compound exercises and get on a diet they're confused.

Don't get me wrong, you can definitely develop a good sixpack with doing ab exercises, but It won't show until you get to a certain fat percentage. For men it's usually around 6% and for women 12%. To burn fat you want to focus on the bigger musclegroups, especially when you have limited time to spend in the gym. Also keep in mind that you can't outtrain a bad diet, nutrition is key! If your abs are visible but you just want to make them pop a little more you I'd recommend doing weighted ab exercises. Such as cable crunches, weighted leg raises, and the abmachine. You need to treat your abs as any other muscle in your body, if you want 'm to grow you need to put some weight on 'm.

To work on your serratus and obliques you can do russian twists and t-bar twists.